Pretreatment effect on inactivation of Escherichia coli O157:H7 ...

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S.N. Albright et al. / Lebensm.-Wiss. U.-Technol. 36 (2003) 381–389
2.0 log cfu/cm 2 with SMAC and TSA, respectively.
These data do not support the findings of Masuda,
Hara-Kudo, and Kumagaf (1998) indicating that soy
sauce (the major component in the marinade) has
negative <strong>effect</strong>s on E. coli O157:H7. Differences seen
in the two studies may have been due to differences in
soy sauce composition or other variables. In the present
study, while dipping the slices in simmering water
achieved bacterial reductions of 2.4–3.1 log cfu/cm 2 ,
the cells appeared to be able to recover during the
marinating step (4 C, 24 h). It is possible that the jerky
marinade (Reynolds & Williams, 1993) supplied the
environment necessary for the inoculated cells to recover
and multiply.
Pre-drying step 2 of S-HP treatment (slices dipped in
78 C pickling brine for 90 s) achieved additional
reductions ðPo0:05Þ of 2.3 and 4.0 log cfu/cm 2 with
SMAC and TSA, respectively; net reductions for both
pre-drying steps were 3.1 and 4.1 log cfu/cm 2 , respectively.
Pre-drying step 2 of VW-M treatment (slices
marinated at 4 C for 24 h) resulted in no significant
bacterial reduction/change (+0.1 and 0.2 log cfu/cm 2
with SMAC and TSA, respectively) and resulted in total
reductions of 0.7 and 0.8 log cfu/cm 2 . When the predrying
steps were reversed (M-VW), the second step,
dipping slices in 57.5 C vinegar/water, resulted in
significant ðPo0:05Þ bacterial reductions of 0.8 and
0.7 log cfu/cm 2 with SMAC and TSA, respectively;
corresponding total pre-drying reductions were 0.7 and
0.5 log cfu/cm 2 . These results do not agree with those of
Entani, Asai, Tsujihata, Tsukamoto, and Ohta (1998),
who observed a 3 log decrease in viable cell numbers of
EHEC O157:H7 NGY-10 after 50 s of dipping in a 2.5%
acetic acid solution held at 50 C. It may be that that
shorter exposure time to the 2.5% acetic acid solution
used in the present study (57.5 C, 20 s) limited the <strong>effect</strong>
of the acetic acid. Entani et al. (1998) reported greater
inactivation of E. coli O157:H7 at higher temperatures,
higher concentrations of acetic acid and with longer
exposure time. Further work is needed to determine if
using a higher concentration of acetic acid, higher
dipping temperature and/or longer dipping time would
have increased bacterial inactivation on jerky products.
3.3. Effects of drying
After 4 h of drying, the BW-M treatment achieved
additional reductions of 2.5 and 2.2 log cfu/cm 2 with
SMAC and TSA, respectively; corresponding total
reductions were 3.8 and 4.2 log cfu/cm 2 (Table 1). The
S-HP treatment method achieved bacterial reductions
(2.5 and 1.7 log cfu/cm 2 with SMAC and TSA, respectively)
during the first 4 h of drying that were similar to
the BW-M treatment, but corresponding total reductions
(5.6 and 5.8 log cfu/cm 2 ) were higher given the
greater <strong>effect</strong>iveness of the S-HP pre-drying treatment.
The VW-M and M-VW treatments achieved additional
reductions of 3.6–4.2 cfu/cm 2 during the first 4 h of
drying, depending on treatment and agar media, with
total reductions at 4 h of 4.4–4.9 log cfu/cm 2 .
Very little bacterial destruction occurred between 4
and 10 h of drying for any of the treatments (0.0 and
0.8 log cfu/cm 2 ). For the BW-M treatment, total reductions
after 10 h of drying were 4.5 and 4.3 log cfu/cm 2 for
SMAC and TSA, respectively (Table 1). Total reductions
were somewhat higher for the two vinegar/watermarinade
treatments (4.7–5.2 log cfu/cm 2 , respectively),
depending on treatment order and culture media. The
S-HP treatment resulted in the highest total bacterial
reduction after 10 h of drying (5.7–5.8 log cfu/cm 2 ).
Data of this study are in partial agreement with
results from Harrison and Harrison (1996) and Harrison
et al. (1998). Harrison and Harrison (1996) found that
pre-heated (71.1 C) and unheated beef strips had a 5-log
reduction when dried at 60 C (140 F) for 10 h in a
home-type dehydrator. In this study, meat seasoned for
24 h, then heated in a salt–sugar brine at 78 C (172 F)
for 90 s and dried for 10 h at 62.5 C (145 F) achieved
X5-log reduction (5.7–5.8 log cfu/cm 2 ). However, beef
slices pre-heated in boiling water at 94 C (203 F)
for 15 s, then marinated before drying did not achieve
a 5-log reduction (4.3–4.5 log cfu/cm 2 ). Differences in
time and temperature between the two studies could
have accounted for differences in reductions of bacterial
populations. In addition, the marinating step (4 C, 24 h)
following the boiling water dip could have allowed
sufficient time for the heat-shocked bacteria to regain
resistance.
Harrison et al. (1998) evaluated ground beef jerky
prepared with a spice (ingredients not listed)/cure (salt
and sodium nitrite) mix and dried at 60 C (140 F) for
either 6 or 8 h. The authors indicated that reductions of
5.2 logs cfu/g were achieved in pre-heated and unheated
jerky containing the spice and cure mix. In this study, a
cure mix was not evaluated, but the S-HP treatment
relied heavily on spices (salt, sugar and black pepper) in
both steps and it was the only treatment that achieved
X5-log reduction following drying. It is presumed that
the combination of pre-drying steps 1 and 2, seasoning
with pickling spices (4 C, 24 h) and dipping in hot
pickling brine (78 C, 90 s), were responsible for the
significant ðPo0:05Þ bacterial reductions seen. It should
be noted that previous research has not been done at
higher elevations (4900 ft) or lower relative humidity;
thus, these geographical factors (elevation and humidity)
may account for some of the differences seen.
For all treatments, populations of surviving bacteria
reached a plateau after 4 h of drying. Even after 10 h of
drying, a residual population of cells was not destroyed
(0.8–2.7 log cfu/cm 2 ). Perhaps, there was no further destruction
of bacteria due to casehardening (McWilliams,
1993) occurring on the surface of the jerky slices when